A cellular communication system comprises a number of base stations, each of which uses one or more antenna elements to serve one or more cells (geographic regions). The base station functions to communicate with mobile communication devices (MCDs) (e.g., smartphones, tables, phablets, etc.) to provide the MCD with access to a network (e.g., the Internet or other network). A base station typically includes at least an antenna element, a radio unit (RU) and a digital unit (DU). An RU typically includes a receiver and a transmitter in order to transmit data to and receive data from an MCD.
The signal transmitted by a base station may be received by an MCD with poor quality when the MCD is in certain locations. For example, when an MCD is located inside of a building (e.g., an office building, a home) and the base station antenna that is serving the MCD is located outside of the building, the MCD may not receive a strong signal from the antenna. Likewise, in such a situation, in order for the MCD to transmit a signal to the base station, the MCD may have transmit the signal using more power than the MCD would have had to use had the MCD been located outdoors. Such a situation reduces the MCD's battery life.
A solution to this problem is to install small transceiver units (a.k.a., “radio heads”) indoors and connect each of the radio heads to an RU of a base station using a cable (e.g., local area network (LAN) cable, such as an Ethernet cable). For example, in a large building with poor network coverage, a radio head connected via a cable to an RU may be placed on each floor of the building. Such a radio head receives via the cable a signal transmitted from the RU and then retransmits the signal wirelessly so that the signal will be received with good quality by an MCD located in the vicinity of the radio head. Likewise, when the MCD transmits a wireless signal, the signal is picked up by the radio head in the vicinity of the MCD and retransmitted by the radio head to the RU via the cable. In this way, a base station can provide good indoor coverage. Accordingly, radio heads include one or more antenna elements for broadcasting and receiving wireless signals, and radio heads may also include amplifiers so that a received signal (e.g., a signal from an RU or a wireless signal from an MCD) can be amplified before it is retransmitted.
One such commercial solution is the Ericsson “Radio Dot System” (RDS). In an RDS, multiple (e.g., one to eight) radio heads are each connected to an RU via an Ethernet cable. The radio heads receive power as well as the communication signals via the cable. In the downlink, each such radio head transmits with a maximum power of 100 mW. Power amplifiers are located in the radio head.
Emergency positioning needs (e.g., E-911) and other location services (LCS) require the position of an MCD to be determined within certain horizontal and vertical accuracies. For example, emergency position requirements may require horizontal inaccuracy to be below 50 meters. Additionally, the vertical inaccuracy requirement has recently been tightened to 3 meters in North America in order to better distinguish between floors in buildings.
Accordingly, there is a need for an improved system for determining the location of an MCD.